Pharmacology and Medication Dosage Calculations

Exam Tip: If a question asks which route does not require an absorption period because the drug is immediately systemic, the answer is intravenous (IV).

Clinical Concepts and Drug Effects

When administering medications, clinicians must be aware of how patients respond to different doses and the legal frameworks governing drug use.

Dose-Response Relationships

Not all drugs follow a linear path of increasing effectiveness. The ceiling effect occurs when increasing a drug dose no longer improves the desired response. Additionally, clinicians must distinguish between drug dependence—a state where a patient feels they cannot function without the medication—and other outcomes of drug overuse like addiction or tolerance.

Legal Classifications and Regulation

The Food and Drug Administration (FDA) is the primary agency responsible for regulating legend drugs (prescription only), over-the-counter (OTC) drugs, and ensuring drugs are safe and effective before they reach the market. The FDA also issues boxed warnings (formerly known as black-box warnings) to alert healthcare providers to serious postmarket problems.

Under the Controlled Substances Act, medications are categorized by their potential for abuse. For example, benzodiazepines are classified as C-IV controlled substances.

Medication Dosage Calculations

Precision is the most vital skill in medication administration. Errors in calculation can lead to subtherapeutic dosing or toxicity.

Fundamental Formulas

The most common method for calculating a dose is the "Desired over Have" formula:

$\text{Amount to Administer} = \frac{\text{Desired Dose}}{\text{Hand (Stock Strength)}} \times \text{Vehicle Volume}$

Example 1: Liquid Concentration A physician orders Pepcid (famotidine) 20 mg. The available stock is 40 mg per 5 mL. $\frac{20\text{ mg}}{40\text{ mg}} \times 5\text{ mL} = 0.5 \times 5\text{ mL} = 2.5\text{ mL}$

Example 2: Converting Grams to Milligrams If a dose of 0.25 g is ordered, you must first convert it to milligrams to match the stock strength: $0.25\text{ g} = 250\text{ mg}$

Concentration and Percentage Conversions

Understanding ratios is essential for calculating solution strengths. A 1:20 solution is equivalent to a 5% concentration. Similarly, a 1:100 solution contains 1 g of active ingredient in 100 mL, which is a 1% concentration.

Calculating Days' Supply and Quantity

Clinicians often need to determine how long a prescription will last or the total amount to dispense.

Example: Total Quantity An order for amoxicillin is 500 mg tid (three times a day) for 10 days. The stock is 250 mg / 5 mL:

1. Calculate total mg needed: $500\text{ mg} \times 3\text{ (tid)} \times 10\text{ days} = 15,000\text{ mg}$
2. Calculate total mL needed: $\frac{15,000\text{ mg}}{250\text{ mg}} \times 5\text{ mL} = 300\text{ mL}$
3. Determine bottles: If bottles are 80 mL, then $300 / 80 = 3.75$, meaning 4 bottles must be dispensed.

Percentage of Error

To ensure tablet potency, the percentage of error can be calculated by comparing the actual amount to the labeled amount: $\text{Percentage Error} = \frac{|\text{Actual} - \text{Labeled}|}{\text{Labeled}} \times 100$ If a 50 mg tablet actually contains 48 mg: $\frac{|48 - 50|}{50} \times 100 = \frac{2}{50} \times 100 = 4\%$

Prescription Components and Communication

A valid prescription must contain specific elements to ensure patient safety. In a retail setting, this is called a prescription, while in an inpatient setting, it is referred to as a medication order.

Essential Elements

A complete prescription typically includes:

• Inscription: The name and strength of the drug.
• Signa (Sig): The directions for use.
• Prescriber Signature: The legal authorization.

Standardized Abbreviations

To prevent errors, healthcare providers use standardized abbreviations. Common examples include:

• tid: Three times a day.
• qid: Four times a day.
• q6h: Every 6 hours.
• pc: Before meals.
• sc: Subcutaneous.
• DAW: "Do Not Allow Substitutions" (used when a generic cannot be used).

Warning: Always verify abbreviations. For instance, OD refers to the right eye, while OS (not listed here but common) refers to the left eye.

Documentation and Safety

Proper documentation and the use of auxiliary labels are vital. Auxiliary labels are used to communicate specific instructions regarding the taking or storing of a medication. Furthermore, the National Drug Code (NDC) is used to identify the specific manufacturer, product, and container size.